1. Field of the Invention
The invention relates to a new liquid crystal compound useful as a material for a liquid crystal display device, and a liquid crystal composition comprising the compound. More specifically, the invention relates to a new liquid crystal compound having a low viscosity, a good compatibility with other liquid crystal compounds, an appropriate refractive index anisotropy value and dielectric anisotropy value, and capable of obtaining a steep electrooptical characteristic when used for a liquid crystal display device, and to a liquid crystal composition comprising the compound and a liquid crystal display device comprising the liquid crystal composition.
2. Related Art
In liquid crystal display devices, a classification based on the operation mode of liquid crystals includes phase change (PC), twisted nematic (TN), super twisted nematic (STN), bistable twisted nematic (BTN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA) and so forth. A classification based on the driving modes of devices includes a passive matrix (PM) and an active matrix (AM). The PM is further classified into static, multiplex or the like, and the AM is classified into a thin film transistor (TFT), metal-insulator-metal (MIM) or the like.
These liquid crystal display devices comprise liquid crystal compositions having appropriate physical properties. The liquid crystal compositions preferably have appropriate physical properties for improving the characteristics of the liquid crystal display devices. General physical properties necessary for a liquid crystal compound which is a component of the liquid crystal compositions are as follows:    (1) being chemically stable and physically stable;    (2) having a high clearing point (the phase transition temperature of a liquid crystal phase to an isotropic phase);    (b 3) being low in the minimum temperature of liquid crystal phases (a nematic phase, a smectic phase and so forth), especially in the minimum temperature of the nematic phase;    (4) having a low viscosity;    (5) having an appropriate optical anisotropy;    (6) having an appropriate dielectric anisotropy; and    (7) having an excellent compatibility with other liquid crystal compounds.
A voltage holding ratio can be increased by using a composition containing a chemically and physically stable liquid crystal compound as described in item (1) for a display device.
The temperature range of a nematic phase can be widened by using a composition containing a liquid crystal compound having a high clearing point or the low minimum temperature of liquid crystal phases as described in items (2) and (3), and thus a display device can be used in a wide temperature range.
Response speed can be improved by using a composition containing a compound having a small viscosity as described in item (4) for a display device. The contrast of a display device can be improved in the case of the display device using a composition containing a compound having an appropriate optical anisotropy as described in item (5).
Further, when a liquid crystal compound having a large dielectric anisotropy is used, the threshold voltage of a liquid crystal composition containing this compound can be decreased, the driving voltage of a display device can be decreased, and power consumption can be reduced.
A liquid crystal compound is generally used as a composition prepared by being mixed with many other liquid crystal compounds in order to exhibit characteristics which can be hardly achieved with a single compound. Thus, the liquid crystal compound used for a display device preferably has a good compatibility with other liquid crystal compounds and so forth, as described in item (7).
Conventionally, various compounds having a trifluoroalkyl, trifluoroalkenyl, and trifluoroalkynyl group at side chains have been synthesized as liquid crystal compounds which can be used preferably for liquid crystal display devices such as TN, STN, and TFT, and some of them are used practically.
For example, Patent document 1 discloses a compound represented by formula (ω-1) as a compound having a trifluoroalkyl group at the side chain. However, the compound (S-1) has a small dielectric anisotropy when mixed into a liquid crystal composition.
Patent documents 1, 2, and 3 disclose compounds represented by formulas (S-2) to (S-4) as compounds having a trifluoroalkenyl group at the side chains. However, none of the compounds has a sufficiently large dielectric anisotropy when mixed into a liquid crystal composition. The compound (S-3) does not have a sufficient stability to heat or light. The clearing point of the compound (S-4) is low when mixed into the liquid crystal composition.
Patent document 1 further discloses a compound represented by formula (S-5) as a compound having a trifluoroalkynyl group at the side chain. However, this compound neither has a sufficiently large dielectric anisotropy when mixed into a liquid crystal composition.
Patent document 4 discloses a compound represented by formula (S-6) as a compound having a styrene skeleton. However, the compound (S-6) does not have a large dielectric anisotropy when mixed into a liquid crystal composition.
Patent document 5 discloses a compound represented by formula (S-7) as a compound having a fluorine-substituted styrene skeleton. However, the compound (S-7) has a narrow range (mesophase range) for exhibiting liquid crystallinity, and a low clearing point when mixed into a liquid crystal composition.
Patent documents 6 and 7 disclose compounds represented by formulas (S-8), (S-9), and (S-10) as trifluorobenzene derivatives. However, all the compounds have a narrow range (mesophase range) for exhibiting liquid crystallinity, and a low clearing point and a insufficiently large dielectric anisotropy when mixed into liquid crystal compositions.
Further, Patent documents 8 to 11 and Non-patent documents 1 to 6 describe methods of forming a trifluoroalkenyl group or trifluoroalkynyl group, or reactions employing the groups as a precursor. However, none of the documents aim at use as a liquid crystal compound, and disclose the structure and characteristics of such compounds.

Patent documents cited are No. 1: WO 1990/013610 A; No. 2: JP H7-138196 A/1995; No. 3, JP 2005-298466 A; No. 4: JP H10-95977 A/1998; No. 5: WO 1992/021734 A; No. 6: JP H2-233626 A/1990; No. 7: WO 1991/013850 A; No. 8: JP S58-92627 A/1983; No. 9: WO 2004/058723 A; No. 10: JP S57-54124 A/1982; and No. 11: WO 1990/09972 A.
Non-patent documents cited are No. 1: Chemistry Letters (2005), 34 (12), 1700-1701; No. 2: Tetrahedron (2004), 60 (51), 11695-11700; No. 3: Tetrahedron (2003), 59 (38), 7571-7580; No. 4: Synthesis (1981), (5), 365-366; No. 5: Bulletin of the Chemical Society of Japan (1999), 72 (4), 805-819; and No. 6: Bulletin of the Chemical Society of Japan (1989), 62 (4), 1352-1354.
The invention concerns a compound represented by formula (1):
wherein R is hydrogen, alkyl having 1 to 12 carbons, alkenyl having 2 to 12 carbons, alkoxy having 1 to 11 carbons, or alkenyloxy having 2 to 11 carbons;
ring A1, ring A2, and ring A3 are each independently 1,4-cyclohexylene, 1,4-phenylene, 1,4-cyclohexenylene, 1,3-dioxane-2,5-diyl, tetrahydropyran-2,5-diyl, pyrimidine-2,5-diyl, pyridine-2,5-diyl, naphthalene-2,6-diyl, decahydronaphthalene-2,6-diyl, or 1,2,3,4-tetrahydronaphthalene-2,6-diyl, and in these rings, hydrogen may be replaced by halogen;
Z1, Z2, and Z3 are each independently a single bond, —CH2CH2—, —CH═CH—, —C≡C—, —COO—, —OCO—, —CF2O—, —OCF2—, —CH2O—, —OCH2— —CF═CF—, —(CH2)4-, —(CH2)2CF2O—, —OCF2(CH2)2—, —CH═CH(CH2)2—, or —(CH2)2CH═CH—;
L1 and L2 are each independently hydrogen or halogen, and at least one of L1 and L2 is halogen;
W is —CH═CH— or —C≡C—; and
n and m are each independently an integer of 0 to 2, and the sum of n and m is an integer of 0 to 3.
The invention also concerns a liquid crystal composition comprising at least one compound. The invention also concerns a liquid crystal display device comprising the liquid crystal composition, and so forth.